Refraction is a phenomenon which causes light rays to be bent as they are transmitted from a material of one refractive index to a material having a different index. The relative differences in refractive index within a piece of glass are a particularly important property, and must be closely controlled. These relative index differences are typically measured with laser interferometry where the index difference must be tightly controlled. Glasses requiring tight control of refractive index variation are widely used in instruments ranging from simple magnifying glasses to complex equipment, such as photolithography lens systems.
It is common practice to employ a liquid of known refractive index while measuring the relative refractive index differences with samples of optical glass. Such index-matching liquids are commercially available, and are known as optical immersion liquids. Index-matching fluids are used to fill the gap between polished glass reference flats and a rough-ground test surface. They eliminate the need to polish every test surface prior to measurements. With the test sample in the matching liquid, light is successively passed through the first reference flat, the index-matching liquid, the test sample, the index-matching liquid, and the second reference flat. A recording of the laser interference fringe pattern produced thus determines whether the desired glass quality is obtained.
It is necessary to obtain a high degree of homogeneity in optical glass during the glass production processes. Thus, the presence of inclusions or foreign materials, such as contaminants, must be avoided. Also, streaks, known as striae or cords, which may result from the glass forming, melting and/or mixing processes, are a cause for rejection. Homogeneity can be readily checked since the defects will have a different refractive index from the glass. The need for frequent checking of glass homogeneity, and for a reliable immersion fluid for this purpose, are apparent.
It has been found that variations in optical immersion liquids can occur between lots, and also due to deterioration and/or contamination on standing. Also, there are often differences in environmental conditions, such as temperature, between the usage area and the test area at which the liquid was qualified. The application in which the liquid is used may also be using a different wavelength of light than the wavelength at which the liquid was qualified. This will result in a shift of the observed refractive index of the immersion liquid between test locations.
Slight differences in index match between the immersion liquid and glass, coupled with non-flat test surfaces on the test sample, can cause significant errors in homogeneity measurements. Errors due to finishing/separation of the glass sample are commonly circularly symmetric about the disc radius. Consequently, immersion oil mismatch will cause circularly-symmetric errors in the transmitted wavefront measurement. This is especially evident in polynomial wavefront fits.
It has been shown that immersion liquids that do not have a desired refractive index may be mixed with other compatible immersion oils of different index to optimize the index of the mixture. Therefore, it would be desirable to have a quick, easy way to establish a proper mixture ratio using small amounts of immersion liquid.
It is then a basic purpose of the invention to provide a device and method to meet these needs. A specific purpose is to provide a quick, easy method of checking the relative refractive index match between an optical immersion liquid and the glass sample. Another specific purpose is to provide a means of quickly deciding a correction ratio to use in mixing a bulk quantity of optical immersion liquid to obtain a desired refractive index. A further specific purpose is to provide a quick check of refractive index match that does not delay production schedules.